1. Field of the Invention
The present invention is related to optical switches, and, more particularly to switching input light between output paths in an optical switch using a heater.
2. Description of the Related Art
An optical switch is a device which switches input light between output optical paths. For example, when the input light enters the optical switch from an input fiber, the light exits the optical switch through one of two output fibers. This is a 1-input 2-output switch or 1 by 2 switch, which is the most fundamental switch. There are more complicated switches, such as 2 by 2, 1 by N, N by N switches, which are realized by combining several 1 by 2 switches.
The most common method of switching the light path (the path which the input light follows through the optical switch) is a mechanical one. The switch has a moving part, such as a prism, a mirror, or a piece of fiber, that is mechanically moved to switch the light path. The mechanical switches have a drawback of slow switching and being less reliable in switching operations. For this reason, researchers have attempted to develop a non-mechanical switch. One practical non-mechanical switch is a magneto-optical switch, which is an optical switch using Faraday rotation and which was commercialized by FDK. Another type of non-mechanical switch uses a liquid crystal, which has a problem in the reliability of the liquid crystal itself.
FIG. 1 shows an example of a bulk switching device 10 of the prior art based upon a Mach-Zehnder interferometer. Mach-Zehnder interferometers are well-known in the art. The bulk switching device 10 receives input light A. Input light A is then split between paths A.sub.1 and A.sub.2 using conventional 50/50 coupler (or splitter) 12, which directs half of the power of input light A to travel along path A.sub.1 and the other half to travel along path A.sub.2. The 50/50 coupler is also referred to as a 3-dB coupler.
The portion of the input light A following path A.sub.1 is reflected off of mirror 14 to 50/50 coupler (or splitter) 22. The portion of the input light A following path A.sub.2 travels through glass block 16 (on top of which heater 18 is situated) and is reflected off of mirror 20 to 50/50 coupler (or splitter) 22. Then, 50/50 coupler 22 recombines the light arriving from path A.sub.1 and the light arriving from path A.sub.2 into output light B and C. The power included in output light B and in output light C is dependent upon the relative phase between the light arriving from path A.sub.1 and the light arriving from path A.sub.2, that is, whether the light from path A.sub.1 is in-phase (the phases of the light traveling in A.sub.1 and A.sub.2 differ by 0 radians or an integer multiple of 2.pi. radians), out-of-phase (the phases of the light traveling in A.sub.1 and A.sub.2 differ by .pi./2 radians or an odd-number multiple of .pi./2 radians), opposite phase (the phases of the light traveling in A.sub.1 and A.sub.2 differ by .pi. radians or an odd-number multiple of .pi. radians), etc., with the light from path A.sub.2. Accordingly, the input light A is switched between output path B and output path C.
The bulk switching device 10 is an optical switch which switches output paths based upon temperature change. More particularly, the relative phase between the light arriving at 50/50 coupler 22 from paths A.sub.1 and A.sub.2 can be changed by glass block 16 by using heater 18 to heat the glass block 16 and change the refractive index of the glass block 16, thus switching the output light path between output path B and C. However, the temperature changes are very slow because the glass block 16 is relatively large, on the order of 5 mm.times.5 mm or 1 cm. Therefore, minutes are required to switch the output light between paths B and C, which is too slow to be practical in lightwave systems. FIG. 2 shows a cross-section of glass block 16 with heater 18 in the prior art. Glass block 16 includes a light beam 24 through which light traveling along path A.sub.2 passes.
Because of its slow speed, the bulk switching device 10 is not of practical use as an optical switch.
Currently, there is work taking place on non-mechanical switches using optical waveguides. But the optical waveguide switches are not yet in use because of their large insertion loss and crosstalk.
An example of a waveguide switch of the related art is shown in FIG. 3A, which shows a Mach-Zehnder interferometer switch 26. As shown in FIG. 3A, when input light A traveling in fiber 27 enters an input waveguide 28 formed on the surface of a glass (or another crystal) substrate 30, the input light A is split into two arms A.sub.1 and A.sub.2 of the waveguide 28. The light traveling in arms (or paths) A.sub.1 and A.sub.2 is then combined into one of the two output waveguides B or C, each of which is coupled to a respective fiber. Here, the output light can travel into one of the two waveguides B or C, depending on the optical phase difference between the light traveling through the two arms A.sub.1 and A.sub.2. If one of two arms, for example A.sub.2, is heated with a heater 32, the temperature change of the waveguide A.sub.2 changes the refractive index of path A.sub.2 which causes a change in optical phase of the light traveling through A.sub.2. Thus, the output light path is switched between B and C by an electric current input into the heater 32. The heater 32 is, for example, a metal coating on the glass surface 30, and is attached to the waveguide A.sub.2.
A cross section 34 of the substrate 30 which includes heater 32 is shown in FIG. 3B. As shown in FIG. 3B, the heater 32 is placed on the surface S of the substrate 30 and to the side of waveguide A.sub.2, and heats an area 36 which includes waveguide A.sub.2. The area 36 is typically 20-30 micrometers in diameter, while the diameter of the waveguide A.sub.2 is typically 10 micrometers, which means that the temperature change in the waveguide A.sub.2 effected by the heater 32 is very quick, and, accordingly, the switching speed of the switch 26 is very fast. However, since the waveguide A.sub.2 is formed on the surface of the substrate 30 and is not enclosed by the substrate 30, the waveguide A.sub.2 is asymmetric, which affects the polarization of the light traveling within the waveguide A.sub.2. The polarization of the light traveling in the waveguide A.sub.2 is affected in that the horizontal component of the light and the vertical component of the light may each be traveling at different speeds, and may, therefore, have different optical phases when they reach the end of the waveguide A.sub.2. Accordingly, the horizontal oscillation of the light traveling in waveguide A.sub.2 may be different than the vertical oscillation of the light traveling in waveguide A.sub.2, and the proper interference between the light traveling in arm A.sub.1 may not occur with the light traveling in arm A.sub.2, thus producing incorrect output from the interferometer (or switch) 26.
Since the switch 26 is made on a glass or other crystalline substrate, it is very expensive to make. The switch 26 is also difficult to make. In addition, the switch 26 is polarization dependent, as discussed herein above, with the polarization state of the light traveling through a fiber being unable to be maintained along the fiber for a long distance, such as 1/2-1 meter or longer.
The switch 26 shown in FIG. 3A is explained in "Low-Power Compact 2.times.2 Thermooptic Silica-on-Silicon Waveguide Switch with Fast Response", by Q. Lai, W. Hunziker, and H. Melchior, IEEE Photonics Technology Letters, Vol. 10, No. 5, pp. 681-683, May 1998.
Further, and generally, optical waveguide devices of the related art also experience problems coupling to input and output fibers.
Also known in the art is a Mach-Zehnder interferometer using optical fibers and 3-dB couplers. The maintenance of the polarization state of the light traveling in an optical fiber is one of the most critical problems in most fiber interferometers using normal (conventional) fibers, not birefringent (polarization maintaining) fibers. Typically, the polarization state of the light traveling through a fiber changes over a distance of 1/2 meter.
A problem is that normal optical fibers cannot maintain the polarization state of the light traveling through them for longer distances, such as 1/2-1 meter. Therefore, if the polarization state of the input light to a Mach-Zehnder interferometer using optical fibers is (for example) vertical, then, after traveling through a first 3-dB coupler, through 2 arms including optical fibers for a longer distance, then to a second 3-dB coupler, the light from either or both of the 2 arms may not have maintained vertical polarization. Accordingly, the light from the 2 arms may not properly interfere with each other at the output of the Mach-Zehnder interferometer, the light may not be transmitted to the proper output path, and some light may be transmitted to an improper output path. Birefringement (polarization-maintaining) fibers are known in the art and are commercially-available, but include disadvantages themselves.
The structure of 3-dB couplers of the related art is shown in FIGS. 4A and 4B. The 3-dB coupler 36 shown in FIG. 4A is made by melting two fibers after the fibers are twisted. The 3-dB coupler 38 shown in FIG. 4B is made by contacting two fibers after each of the fibers are shaved on one of their surfaces.
Michelson interferometers are also known in the art.
Optical circulators, which are well-known in the art, are non-reciprocal devices (being implemented typically using a magnetic field), and have several input/output ports, with light entering a given port and being circulated typically in a clockwise manner to the next adjacent output port.